Solid state wavelength converters that convert an image encoded in the intensity of light characterized by a first wavelength to the image encoded in light characterized by a second wavelength are known in the art. The light characterized by the first and second wavelengths is hereinafter referred to as “input light” and “output light” respectively and for clarity and simplicity of presentation it is assumed that the encoded image is an image of an object.
One type of wavelength converter comprises a layered body comprising a plurality of thin contiguous layers formed from optically and/or electrically active materials. One of the layers is a photo-conducting layer and one of the layers is a photo-modulating layer. Material in the photo-conducting layer absorbs energy from light that passes through the photo-conducting layer and converts the absorbed energy to electron-hole pairs. Material in the photo-modulating layer modulates a characteristic, generally intensity, of light that passes through the photo-modulating layer by an amount that depends upon the strength of an electric field in the modulating layer. Generally, the outermost layers in the layered body are formed from a transparent conducting material and function as electrodes. An appropriate electrical power supply is connected to the electrodes.
When the wavelength converter is in operation, the power supply applies a potential difference between the electrodes and generates thereby an electric field in the layers. Light, i.e. input light, from an object to be imaged is focused onto the photo-conducting layer. In the photo-conducting layer the input light has a spatially varying intensity corresponding to the image of the object. Photons in input light are absorbed in the photo-conducting layer and generate electron-hole pairs in the layer. The number of electron-hole pairs produced in a region of the photo-conducting layer is substantially proportional to the intensity of the input light in the region. Thus, the varying intensity pattern, hereinafter referred to as an “input image”, of the input light in the photo-conducting layer is “copied” into the density distribution of the generated electron-hole pairs.
Under the influence of the electric field generated by the power supply, the electrons from the electron-hole pairs drift in the direction of the photo-modulation layer and are trapped near to or on a surface of the photo-modulation layer. The trapped electrons generate an electric field, hereinafter referred to as a “modulation field”, in the photo-modulation layer. Since the density distribution of electron-hole pairs generated in the photo-conducting layer images the object, the trapped electron density distribution, hereinafter referred to as a “charge image”, and the magnitude of the modulation field generated by the trapped electrons also image the object.
After exposure to the input light, the converter is exposed to light, i.e. output light, radiated from an appropriate light source. The output light is caused to be incident on the wavelength converter and exits the wavelength converter after passing through the photo-modulation layer. The photo-modulation layer modulates the output light responsive to the magnitude of the modulation field. Output light that passes through a region of the modulation layer in which the modulation field is strong is strongly modulated. Output light that passes through a region of the modulation layer in which the modulation field is weak is weakly modulated. Therefore, upon exiting the converter, the output light is coded with the image of the object, i.e. the input image of the object is copied into the output light, and the output light may thereafter be processed to provide an image of the object.
U.S. Pat. No. 5,124,545 to Takanashi et. al. describes a number of different wavelength converters of this type. One wavelength converter described in the patent comprises a photo-conducting layer formed, for example from Cadmium Sulfide (CdS) or Bismuth Silicon Oxide (B12SiO20), that is contiguous with a photo-modulation layer, “such as a single crystal of lithium-niobate or a nematic liquid crystal”. Both the input light and the output light pass through both layers. The energy of photons in the output light is therefore chosen to be less than the band-gap energy of the photo-conducting layer. This prevents the output light from generating electron-hole pairs in the photo-conducting layer that would destroy correspondence between an input image of an object being imaged with the wavelength converter and a charge image of the object in the wavelength converter that generates a modulation field. As a result, a prior art wavelength converter of this type is generally used when energy of photons in the input light is greater than energy of photons in the output light. A wavelength converter of this type would be practical for converting a UV input image of an object to an “output” image of the object in the visible spectrum, but not for converting an IR input image of the object to a visible output image of the object.
Another wavelength converter described in the patent comprises a dielectric mirror or a “light insulating film” sandwiched between a photo-conducting layer and a photo-modulation layer. In this wavelength converter, output light is incident on and passes through the photo-modulation layer and is then reflected by the dielectric mirror to pass through the photo-modulation layer a second time and exit the wavelength converter. As a result of the mirror, the output light never reaches the photo-conducting layer and therefore does not affect a charge image of an object being imaged with the converter. A wavelength converter of this type can therefore convert an image encoded in a relatively “low energy” input light to an image encoded in a relatively “high energy” output light, e.g. an IR image to a visible light image.
However, in wavelength converters with a mirror, the presence of the mirror tends to increase the distance between a charge image formed in the wavelength converter and the wavelength converter's photo-modulation layer. In addition it tends to increase the distance, in a direction perpendicular to the plane of the photo-modulation layer, over which the charge image is distributed. Both these effects of the mirror tend to blur or reduce sharpness with which a modulation field generated by a charge image corresponds to an input image of an object being imaged. Therefore, the mirror tends to reduce the spatial resolution of the wavelength converter.
In many prior art wavelength converters, for practical intensities of input light, variations in the density of trapped electrons are often too small to affect acceptable modulation of output light. As a result, the sensitivity of these wavelength converters is not sufficient for many applications. It would be advantageous to have wavelength converters with increased sensitivity.